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Article Reference Article Loop corrections and graceful exit in string cosmology FOFFA, Stefano, MAGGIORE, Michele, STURANI, Riccardo Abstract We examine the effect of perturbative string loops on the cosmological pre-big-bang evolution. We study loop corrections derived from heterotic string theory compactified on a $Z_N$ orbifold and we consider the effect of the all-order loop corrections to the Kahler potential and of the corrections to gravitational couplings, including both threshold corrections and corrections due to the mixed Kahler-gravitational anomaly. We find that string loops can drive the evolution into the region of the parameter space where a graceful exit is in principle possible, and we find solutions that, in the string frame, connect smoothly the superinflationary pre-big-bang evolution to a phase where the curvature and the derivative of the dilaton are decreasing. We also find that at a critical coupling the loop corrections to the Kahler potential induce a ghost-like instability, i.e. the kinetic term of the dilaton vanishes. This is similar to what happens in Seiberg-Witten theory and signals the transition to a new regime where the light modes in the effective action are different and are related to the original ones by S-duality. In a [...] Reference FOFFA, Stefano, MAGGIORE, Michele, STURANI, Riccardo. Loop corrections and graceful exit in string cosmology. Nuclear Physics. B, 1999, vol. 552, p. 395-419 DOI : 10.1016/S0550-3213(99)00248-5 Available at: http://archive-ouverte.unige.ch/unige:2212 Disclaimer: layout of this document may differ from the published version. 1 / 1 IFUP-TH 8/99 February 1999 Loop corrections and graceful exit in string cosmology Stefano Foffaa, Michele Maggiorea and Riccardo Sturanib,a (a) Dipartimento di Fisica, via Buonarroti 2, I-56100 Pisa, Italy, and INFN, sezione di Pisa. (b) Scuola Normale Superiore, piazza dei Cavalieri 7, I-56125 Pisa. Abstract We examine the effect of perturbative string loops on the cosmological pre-big-bang evolution. We study loop corrections derived from heterotic string theory compacti- fied on a ZN orbifold and we consider the effect of the all-order loop corrections to the K¨ahler potential and of the corrections to gravitational couplings, including both threshold corrections and corrections due to the mixed K¨ahler-gravitational anomaly. We find that string loops can drive the evolution into the region of the parameter space arXiv:hep-th/9903008v1 28 Feb 1999 where a graceful exit is in principle possible, and we find solutions that, in the string frame, connect smoothly the superinflationary pre-big-bang evolution to a phase where the curvature and the derivative of the dilaton are decreasing. We also find that at a critical coupling the loop corrections to the K¨ahler potential induce a ghost-like insta- bility, i.e. the kinetic term of the dilaton vanishes. This is similar to what happens in Seiberg-Witten theory and signals the transition to a new regime where the light modes in the effective action are different and are related to the original ones by S-duality. In a string context, this means that we enter a D-brane dominated phase. 1 Introduction Pre-big-bang cosmology has been initially developed using the lowest-order effective action of the bosonic string [1]. This allowed to understand the basic features of this cosmological model: the Universe starts at weak coupling and low curvature, follows a superinflationary evolution, and enters a large curvature phase. As long as we are in the perturbative regime, a description with the lowest order effective action is adequate, and can be used to discuss the cosmological evolution [1], the generality of the initial conditions [2], and even to develop some interesting phenomenological consequences concerning the generation of primordial gravitational, axionic, dilatonic and electromagnetic backgrounds [3]. In particular, the low frequency part of these spectra is only sensitive to the low curvature part of the evolution. At lowest order, the cosmological evolution unavoidably reaches large curvatures and strong coupling and finally runs into a singularity. At this stage it is necessary to go beyond the lowest order effective action for understanding how string theory cures the big-bang singularity, and for understanding the matching of pre-big-bang cosmology with standard Friedmann-Robertson-Walker (FRW) post-big-bang cosmology (the so- called ‘graceful exit’ problem [4-12]). One can imagine two possible scenarios. The first is that pertubative corrections succeed in turning the regime of pre-big-bang accelerated expansion into a decelerated expansion, and at the same time the dilaton is attracted by the minimum of its non-perturbative potential. This should take place before entering into a full strong coupling regime, so that perturbative results can still be trusted. In the second scenario the evolution proceeds toward the full strongly coupled regime. In this case one must take into account that at strong coupling and large curvature new light states appear and then the approach based on the effective supergravity action plus string corrections breaks down. The light modes are now different and one must 1 turn to a new effective action, written in terms of the new relevant degrees of freedom. In particular, at strong coupling D-branes [13] are expected to play an important role, since their mass scales like the inverse of the string coupling, 1/g and they are ∼ copiously produced by gravitational fields [14] (see also refs. [15, 16, 17, 18] for related approaches to singularities in string theory). In this paper we investigate how far one can go within the first scenario, analyzing a variety of perturbative string loop corrections. The motivation comes in part from the works [10, 11], where the authors discuss general properties that the loop corrections should have in order to trigger a successful graceful exit. In particular, they find that the corrections should have the appropriate sign, so to induce violation of the null energy condition (NEC), but there should also be a mechanism that at some stage turns them off, otherwise the continued violation of the NEC produces an unbounded growth in the curvature and dilaton. In ref. [11] they presented a model with loop corrections chosen ad hoc, both in the sign and in their functional form, just for the purpose of verifying explicitly on a toy model the general results of ref. [10], and they found that with appropriate choices a complete exit transition is indeed obtained. A transition to a regime of decreasing curvature was also obtained, with a potential chosen ad hoc, in the second paper of ref. [1]. This shows that string loops can in principle trigger a complete graceful exit, but leaves open the question of whether this actually happens for the corrections derived from at least some specific compactification of string theory. In particular, the results of refs. [10, 11] show first of all that the corrections should have the appropriate sign, and this already is an interesting point to check against real string derived corrections, and furthermore they should also have a rather non-trivial functional form that suppresses them at strong coupling. String loop corrections have been much studied in the literature, especially for ZN 2 orbifold compactifications of the heterotic string [19-34], and we can therefore ask whether, at least in some compactification scheme, they fulfill the non-trivial properties needed for a graceful exit. In particular, corrections to the K¨ahler potential are known at all loops; this will be very important for our analysis, since in order to follow the cosmological evolution into the strong coupling regime, a knowledge of the first few terms of the perturbative expansion is not really sufficient, and one must have at least some glimpse into the structure at all loops. After discussing how far one can go within a perturbative approach, we will be able to shed some light on the question of whether perturbative string theory is an adequate tool for discussing string cosmology close to the big-bang singularity, or whether instead non-perturbative string physics plays a crucial role, as in the scenario discussed in ref. [14]. The paper is organized as follows. In sect. 2 we discuss the effective action of the ′ heterotic string with α and loop corrections for a ZN orbifold compactification. In sect. 3 we use these corrections to study the cosmological evolution; we will introduce various corrections one at the time, to understand better their role, and we will find solutions that, in the string frame, smoothly interpolate between the pre-big-bang phase and a phase of decelerated expansion. In sect. 4 we examine the limit of validity of the perturbative approach. We point out that loop corrections induce an instability in the kinetic term of the dilaton, that vanishes at a critical coupling. Similarly to what happens in the Seiberg-Witten model, beyond this value of the coupling the effective action is more appropriately written in terms of variables related to the original ones by S-duality. In string theory this means that there is the onset of a new regime where the light modes of the effective action are given by D-branes rather than by the massless modes of fundamental strings. Sect. 5 contains the conclusions. 3 2 The effective action with loop corrections We consider the effective action of heterotic string theory compactified to four dimen- sions on a ZN orbifold, so that one supersymmetry is left in four dimensions, and we restrict to the graviton-dilaton-moduli sector. In a generic orbifold compactification there are the (1, 2) untwisted moduli fields Ui and the diagonal (1, 1) untwisted moduli fields Ti (non-diagonal moduli are included in the matter fields).
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